You have two options to install a p/s engine into an MBX5R. 1) Cut out a slot in the chassis to allow for the pull starter to fit through. 2) Make a engine mount spacer to raise the engine higher off the chassis allowing for the pull starter to fit. I have installed a roto start STS 21 in a Hyper 8 and I made custom 1/16" alloy spacers that went in between the chassis and the engine mounts. I recommend making spacers instead of cutting up your chassis. It mostly depends on the engine that will give you the option of making spacers instead of cutting a slot in your chassis (some engines have huge pull starts). Making the engine mount spacers was a simple job and you dont need an entire shop to make them. I just didnt want to mess with my chassis cause in the future I plan to get a b/s engine which wont need any custom spacers.